Modern household appliances such as, for example, audio and video equipment, and commercial equipment have solid state circuits or other sensitive circuits that are subject to damage caused by electrical transients. As used herein, the term appliance refers to any device that receives electrical power. Currently, in an effort to minimize the effects of transient voltages, variations of a class of device known as a Transient Voltage Surge Suppressor (TVSS) are used. Such devices are used in, for example, consumer and industrial electronics, automotive electronics, and telecom systems as well as commercially available surge protector strips.
The main component within a TVSS is typically a voltage clamping Metal Oxide Varistor (MOV). Additionally, other types of voltage clamping devices such as gas tubes or Zener diodes are used. An MOV has a non-linear Voltage and Current (V-I) curve that has a very high resistance when the voltage across it is small compared to its threshold voltage. Conversely, when the voltage across an MOV is high compared to its threshold voltage, the MOV has a low resistance and allows current to pass through it, thus providing a shunt for transient voltages above the MOV's threshold voltage. In such situations, for example, an MOV can be placed between HOT and NEUTRAL conductors, between HOT and GROUND conductors, and/or between NEUTRAL and GROUND conductors. When the MOV threshold voltage is exceeded, such as when a large enough transient occurs, the nonlinear characteristics of the MOV allows a fault current, possibly in the many hundreds of amps (or more) to temporarily flow. The voltage drop of the wiring feeding the MOV, which temporarily carries this fault current, may cause the output voltage seen by the appliance or sensitive circuit to drop in turn.
For example, if an appliance that has an MOV protecting it that is placed across the HOT line and NEUTRAL at the appliance, and a transient voltage occurs, the flow of current may cause a voltage drop across the HOT feeder conductor to the MOV and a corresponding voltage drop across the NEUTRAL conductor that handles the return current. The net effect is that the NEUTRAL voltage at the appliance is no longer zero volts (with respect to GROUND), and thus presents a shock hazard.
If an MOV is placed, instead, between HOT and GROUND and a transient occurs, a similar voltage drop can occur on the GROUND return path. In this situation, the GROUND may float to a higher voltage. For example, on a grounded appliance, where that appliance contains or is connected to a sensitive electronic circuit, such as, for example, a data circuit, the increase in the GROUND voltage can damage the circuit.
The current method of manufacturing cable assemblies such as type NM (nonmetallic), for example, calls for equal resistance in all conductors (HOT, NEUTRAL, and GROUND) or for the GROUND conductor to be of a smaller diameter (and having a higher resistance) with respect to the other conductors. For conductors made of the same material, these dimensions will reduce in equal voltage drops on conductors that are equal in diameter, or in a greater voltage rise on the GROUND leg for those cable assemblies with reduced diameter GROUND legs. Thus, current commercially available products are not well suited for use with circuits protected by modern TVSS devices.
An additional limitation of current cabling systems is that the circuit breaker that protects the MOV (the breaker that is part of the residential or commercial wiring) cannot always respond rapidly if the MOV begins to fail. In situations such as these, the MOVs can overheat and may even cause a fire. For example, when a 14 AWG wire is feeding an appliance, and the MOV within the appliance starts to fail, the MOV fault current will largely be a function of the two conductors (HOT and NEUTRAL) that are part of the branch circuit wiring. Under many circumstances, the fault current will be insufficient to cause a typical Thermal-Magnetic (TM) circuit breaker to trip.
Therefore, in order to reduce the risk of electrical shock, fire, and potential damage to equipment where TVSS devices are used, a new type of cabling is needed.